1. Field of the Invention
The present invention relates to a method of depositing a photocatalytically-activated self-cleaning coating on a substrate (e.g. glass sheet or a continuous glass float ribbon), to a method of preventing sodium ion poisoning of the photocatalytically-activated self cleaning coating deposited over a sodium ion containing substrate and to articles of manufacture prepared according to the methods.
2. Description of the Related Art
For many substrates (e.g. glass substrates), it is desirable that the surface of the substrate remain "clean," that is to say free of surface contaminants, e.g. common organic and inorganic surface contaminants. Traditionally, this has meant that such surfaces must be cleaned frequently. This cleaning operation is typically performed manually or by mechanical devices. Either approach is quite labor, time and/or cost intensive. A need exists for substrates having surfaces that are self-cleaning or at least easier to clean, which would eliminate or reduce the need for such manual or mechanical cleaning.
Titanium dioxide (TiO.sub.2) coatings are known to provide a photocatalytically-activated self-cleaning (hereinafter "PASC") surface on a substrate. Publications directed to the formation of a PASC titanium dioxide coating on a glass substrate include U.S. Pat. No. 5,595,813 and "Photooxidative Self-cleaning Transparent Titanium Dioxide Films on Glass", Paz et al., J. Mater. Res., Vol. 10, No. 11, pp. 2842-48 (November 1995). Further, a bibliography of patents and articles relating generally to the photocatalytic oxidation of organic compounds is reported in Bibliography of Work On The Photocatalytic Removal of Hazardous Compounds from Water and Air, D. Blake, National Renewable Energy Laboratory (May 1994) and in an October 1995 update and an October 1996 update.
A presently available method of applying a PASC coating (e.g. a titanium dioxide PASC coating) to a substrate is the sol-gel method. With the sol-gel method an uncrystallized alcohol-solvent-based colloidal suspension (the sol) is spray, spin, or dip coated onto a substrate at or about room temperature. The substrate is then heated to a temperature within the range of about 100.degree. C. to 800.degree. C. (212.degree. F. to 1472.degree. F.), to either bond the PASC coating to the substrate and/or to cause the crystallization of the PASC coating, in order to form a crystallized PASC coating (the gel) on the substrate.
One limitation of applying a sol-gel PASC coating is that the sol-gel coating method is not economically or practically compatible with certain application conditions or substrates. For example, when it is desired to provide a PASC coating on a float ribbon during manufacture thereof, the ribbon may be too hot to accept the sol depending in part, on the solvent used in the sol solution. For many solvents used in sol-gel process, it is required to cool the hot float ribbon to about room temperature before applying the sol, and to reheat the float ribbon to a temperature sufficient to crystallize the sol into a PASC coating. Such cooling and reheating operations require a substantial investment in equipment, energy and handling costs, and significantly decrease production efficiency.
The PASC activity of PASC coatings may be significantly reduced or destroyed if sodium ions are present in the substrate and migrate from the substrate into the PASC coating. This process is known as sodium poisoning or sodium ion poisoning. For many substrates which contain sodium ions, the rate of migration of sodium ions into coatings increases as the temperature of the substrate increases. Thus another limitation of the sol-gel coating method is that reheating the substrate increases the opportunity for sodium ion migration, and in turn, sodium ion poisoning of a PASC coating.
Another limitation of forming PASC coatings by the sol-gel method is the thickness of the coatings e.g. several microns thick. Such thick PASC coatings may have an adverse affect on the optical and/or aesthetic properties of PASC coated articles.
As can be appreciated from the foregoing, a need exists for an article of manufacture having a PASC coating deposited therein and for a method of depositing a PASC coating that does not suffer from the drawbacks known in the art.